1. Field of the Invention
The present invention relates to a sound reproduction apparatus capable of three-dimensional stereophonic sound reproduction and a sound reproduction method for use with the sound reproduction apparatus.
2. Description of the Related Art
Hitherto, as a sound field reproduction system for reproducing a three-dimensional stereophonic sound field, for example, a sound system for reproducing three-dimensional stereophonic sound using two speakers in front of and two speakers behind a listener, that is, a total of four speakers, is known. In addition to the sound reproduction system described above, a three-dimensional sound reproduction system for reproducing a three-dimensional stereophonic sound field using two speakers has also been proposed.
When the sound reception of a concert hall is reproduced in a listening room by using a sound reproduction system for reproducing a stereophonic sound field by using two speakers, a sound signal that is radiated from a sound source, such as a musical instrument, and that arrives at the ears of the audience accompanied with the reverberations of the hall is necessary. It is known that such a sound signal is obtained by picking up sound by using a dummy head microphone such that microphones are mounted at the positions of two ears of a dummy head based on the shape of the head of a human being, that is, by binaural sound pickup.
Examples of binaural sound pickup include a method in which a sound signal that arrives at the ears of the audience is directly picked up by arranging a dummy head microphone in a seat of a concert hall, and a method in which sound is picked up by electrically superposing propagation characteristics from the position of the sound source, which are determined by measurements or simulation, to the ears of a listener onto a signal of a sound source such as a musical instrument (see Japanese Unexamined Patent Application Publication No. 5-115098). In the former case of the sound pickup method for directly picking up sound, the propagation characteristics from the position of the sound source to the ears of the listener are acoustically superposed onto the sound from the sound source.
However, in these sound reproduction systems, since propagation characteristics from the position of the speaker disposed in the listening room to the position of the listener are not necessary, a filter for canceling such propagation characteristics needs to be provided.
In order to transmit the sound to the tympanic membrane, which is recorded by the dummy head microphone, to the tympanic membrane of the listener by using a speaker, as a technology for canceling propagation characteristics from the playback speaker to the listener, technologies, such as TRADIS (True Reproduction of All Directional Information by Stereophony) and OSS (Ortho-Stereophonic System), have been developed. Also, a technology in which a technique for inserting indoor propagation characteristics of a performance as given in a concert hall is combined with the above has been proposed (see Japanese Unexamined Patent Application Publication No. 10-70798).
Japanese Unexamined Patent Application Publication No. 5-30600 discloses a method in which a frequency band that is effective for sound image control is separated by a filter, and sound image control is performed on a signal of the frequency band, so that the amount of computation is reduced. Japanese Unexamined Patent Application Publication No. 7-107598 discloses a method in which signal processing is performed by increasing the number of band divisions, and a sound image having natural expansion is reproduced.
A head-related transfer function, which indicates propagation characteristics from the position of the sound source to the ears of the listener in the binaural sound pickup described above, is also called a head diffraction transfer function, and the propagation characteristics are measured by using the sound source direction (angle) as a parameter.
However, since such a head-related transfer function depends on the head shape and the pinna shape, it differs for each listener. In particular, since the characteristics of the high frequency band have large individual differences, a head-related transfer function that applies to many persons cannot be realized over a wide band.
In order to improve the quality of the reproduction sound image when a sound signal picked up by binaural sound pickup is reproduced, theoretically speaking, it is necessary to optimize the sound pickup device for each listener. More specifically, since the head-related transfer function needs to be measured for each listener and optimized, a sound pickup device that is commercially practical for the general public cannot be constructed.
Accordingly, in order for the head-related transfer function to apply to many listeners, it is considered that superposition is performed by permitting a certain degree of error in order to generalize the head-related transfer function. However, if the head-related transfer function is generalized over a wide band, there is a risk of the sound localization of the stereophonic sound becoming unstable, and the sound image that should originally be perceived as a front sound image is mistakenly perceived as a back sound image, that is, so-called reverse front/back mis-perception occurs.
Variations in the head-related transfer function described above occur due to variations of the head shape and the pinna shape of the listener and due to the relationship with the wavelength of sound waves that arrive from the sound source. For this reason, variations in the head-related transfer function for each listener are small for the low frequency components and are large for the high frequency components.
Therefore, if, during sound pickup, an upper limit is provided for the sound band in which sound is picked up and the sound pickup is performed by targeting only the low frequencies, the head-related transfer function can be generalized. However, in that case, there is a drawback in that an unnatural sound having no high frequency components is generated.
As described above, in the conventional binaural sound pickup, since a head-related transfer function is difficult to generalize (standardize), it is not possible to provide a target sensation of sound localization with a natural sound to a large number of listeners.
The problems, described up to this point, resulting from the individual differences of propagation characteristics from the sound source to the ears of the listener affect not only the reproduction of the above-described propagation characteristics from the position of the sound source to the ears of the listener in the original sound field, but also a canceling filter for canceling propagation characteristics from the playback speaker to the ears of the listener in a listening room.
More specifically, since the characteristics of the canceling filter depend on the propagation characteristics of the listener, strictly speaking, it is necessary to design a canceling filter for each listener. The characteristics of such a canceling filter can also be generalized. However, since an error becomes larger particularly in the high frequency band, there are drawbacks in that the canceling effect is small, and the perception of the sound image direction becomes unstable.